Flexible and easily bendable cable have been widely used as a medium for transmitting electrical signals in a variety of information processing apparatus, which typically include computers. An example of such system is a notebook computer having a "clam shell structure", the computer being provided with a keyboard hinged to a cover and which may also include, among others, a liquid crystal display. It is common practice to use flexible cables to transfer electrical signals, (i.e., video signals) between the main body of the system and the cover. The cover of the computer opens from the main body of the system when the computer is in use, and closes when the computer is not in use. A cable requires a high level of mechanical durability against bending when the flexible cable passes through the hinged portion of the cover since it bends each time the cover opens or closes as represented by XGA (extended Graphic Array). Recently, displays have migrated to finer resolutions. The capacity of the video signals being transmitted have correspondingly risen, thereby increasing the transfer rate of the video signals. For this reason, the manner of transmitting video signals has gone from a conventional parallel transfer mode to a high speed balanced transfer mode, referred to as LVDS (Low Voltage Differential Signal). A high anti-Electromagnetic Interference (EMI) performance and impedance stability are both required for a flexible cable that implements the high speed data transfer, such as LVDS. In addition, mechanical strength, such as durability against bending must also be guaranteed. Finally, it is also desirable that the flexible cable be thin (e.g., no more than 0.5 mm thick), as the notebook computers themselves become thinner.
A flexible cable generally consists of a laminated portion in which both surfaces of a copper foil circuit act as a transmission line interspersed between insulating polyimide films. The polyimide films on both sides are pressed against the copper foil circuit layer by a hot roll using an adhesive which acts as both, a protecting film and an insulating film for the circuit layer.
The structure and the manufacturing process of a conventional flexible cable is now considered with reference to FIG. 4, which illustrates a cross-sectional view of the FPC seen in a direction perpendicular to the direction of the wires configured in the wiring layer. This type of flexible cable is manufactured starting from a single sided substrate, as shown in FIG. 4(a). The single sided substrate consists of a copper foil laminated by a hot roll, and the like, on one side of the substrate insulated by the polyimide film. It is well known by those skilled in the art that single sided substrates of this type have been widely used in industry. A wiring layer is then formed by etching the laminated copper foil to a predetermined pattern, as shown in FIG. 4(b). To protect the wiring layer, the same single sided substrate of FIG. 4 (a) is laminated on the lower layer of the wiring layer (FIG. 4c). The laminated single sided substrate is used as a protective layer. Its use is preferred over a single layer of a polyimide film, particularly since single sided substrates are conventional items readily available in industry. The impedance of the flexible cable is determined generally by the pitch and the width of wires of the wiring layer as well as by the dielectric constant and the size (thickness) of the polyimide film which is a substrate material. However, the impedance may vary when the insulating polyimide film layer contacts another insulating object under normal conditions of usage when the polyimide film layer is exposed. For this reason, a single sided substrate is, likewise, laminated on the upper surface of the substrate to cover the surface with a conductive copper foil layer, as shown in FIG. 4(c). The lamination is done by pasting the bottom surfaces of the substrates with a bonding sheet, and then by applying a hot roll, (FIG. 4c). The upper and lower surfaces are coated with a copper foil or film. Thus, an electromagnetic wave generated by a data signal propagating through the wiring layer at high speed is suitably shielded by the metal film. Further, the impedance characteristic is also stabilized, as described above. However, a copper foil which is exposed to the external environment is susceptible to the danger of peeling off from the polyimide film, or cracking because its mechanical strength is very low. Further, the wiring layer is substantially shielded from the external environment by multiple insulating layers so that the ground-to-ground connection to the frame is very weak. In view of the above, it is necessary for the ground frame portion of the wiring layer to be drilled (or dot punched) as shown in FIG. 4(d). The surface is then copper plated to enhance its grounding characteristics, as illustrated in FIG. 4(e). The upper and the lower surfaces are etched as necessary to form the wiring pattern FIG. 4(f). It may also be advantageous to further coat it with an insulating layer to reinforce the copper foil portion of the surface, as shown in FIG. 4(g). To summarize, prior art cables of the type shown in FIG. 4(g) are characterized by a repeated use of multi-layering the semiconductor substrate.
The flexible cable forming a multi-layered structure shown in FIG. 4(g) may have excellent anti-EMI and impedance characteristics for a media for high-speed transmission of video signals. However, it is not recommended to downsize it to the thickness of the notebook PC because it evidently becomes too thick due to its multi-layered structure. Further, since it is substantially thick, it does not inherently assume the pliableness required for a flexible cable. In addition, the manufacturing process is complicated because of the multi-layering, which naturally results in increased cost.